1) Field of the Invention
The present invention relates to a process for recovering a lower alcohol from a reaction mixture containing a reaction product of catechol with the lower alcohol. More particularly, the present invention relates to a process for recovering a lower alcohol from a reaction mixture containing (i) a reaction product of catechol with a lower alcohol, (ii) a by-product consisting of water, and (iii) non-reacted catechol and lower alcohol, by separating the liquid reaction mixture into (a) a vapor fraction comprising the non-reacted lower alcohol and water and (b) another fraction comprising the reaction product and the non-reacted catechol, in a first distillation step; and by recovering the non-reacted lower alcohol from the vapor fraction in a second distillation step and then in a gas-separating step, using a gas-separating membrane which allows a selective permeation of vapor therethrough.
2) Description of the Related Art
It is known that catechol reacts with a lower alcohol to produce an o-lower alkoxy phenol or lower alkyl catechol in accordance with the following reaction: ##STR1##
wherein R represents an alkyl radical.
The o-lower alkoxy phenol, for example, guaiacol (o-methoxyphenol or methylcatechol) or guethol (o-ethoxyphenol or ethylcatechol), is useful as a material for producing vanillin or ethylvanillin of the formula: ##STR2##
Guaiacol is produced by a reaction of catechol with methyl alcohol, and guethol is produced from a reaction of catechol with ethyl alcohol.
The resultant reaction mixture contains (i) a reaction product of catechol with the lower alcohol, (ii) a by-product consisting of water, and (iii) non-reacted catechol and lower alcohol.
In a conventional process, the collection of the reaction product and the recovery of the non-reacted lower alcohol from the reaction mixture are carried out by using a distillation system in which a plurality of distillation columns are combined in series, as shown in, for example, FIG. 2.
In this conventional process, a liquid reaction mixture comprising (i) a reaction product of catechol with a lower alcohol, (ii) a by-product consisting of water and (iii) the non-reacted catechol and lower alcohol is subjected to a first distillation step to separate a resultant vapor fraction comprising the non-reacted lower alcohol and water from a remaining fraction comprising the reaction product and the non-reacted catechol, which respectively have a higher boiling point than that of the lower alcohol and water, and then the vapor fraction comprising the lower alcohol and water is subjected to a plurality of distillation steps to separate the lower alcohol from water. In those distillation steps, a very large amount of heat energy (for example, steam) must be consumed to separate each of the above-mentioned compounds in the reaction mixture, while maintaining a reflux ratio at a high level in each distillation column.
Also, since the lower alcohol and water forms an azeotropic mixture, it is very difficult to recover the lower alcohol in a high concentration from the azeotropic mixture thereof.
Namely, to recover the lower alcohol in a high concentration from the lower alcohol-water mixture by a distillation method, an azeotropic agent consisting of, for example, benzene for the lower alcohol must be added to the lower alcohol-water mixture to selectively remove water from the mixture, but the use of the azeotropic agent, for example, benzene, results in a raise in the cost and a complicated operation of the recovering step. Also, it is necessary to remove the azeotropic agent from the recovered lower alcohol.